A 6-yr field study assessed the long-term impact of Bt cotton producing the Cry1Ac δ-endotoxin on 22 taxa of foliar-dwelling arthropod natural enemies in Arizona. No chronic, long-term effects of Bt cotton were observed over multiple generations of nontarget taxa. Zero-2 taxa declined significantly in unsprayed Bt compared with non-Bt cotton each year. In contrast, positive control studies showed that insecticide applications for caterpillars and other pests in both non-Bt and Bt cotton had much greater negative effects on 10 taxa. Multivariate principal response curves supported the findings of univariate analyses for the entire natural enemy community, showing no effect of Bt cotton but large and long-lasting negative effects from the use of insecticides. Multi-year analyses provided greater statistical power and indicated significant reductions that averaged 19% in five arthropod predator taxa in unsprayed Bt compared with non-Bt cotton. Most of these reductions were likely associated with reductions in lepidopteran prey. However, results of a companion study examining natural enemy function suggest that these minor reductions in Bt cotton have little ecological meaning. Multi-year analyses showed an average significant reduction of 48% in 13 taxa for plots receiving insecticide applications. On average, a 3-yr study with four replicates per year was sufficient to discern changes of ≈20%, with 80% power in unsprayed cotton. This long-term study indicates that the effects of Bt cotton on a representative nontarget community are minor, especially in comparison with the alternative use of broad-spectrum insecticides. Guidelines for improving nontarget field studies are discussed.
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